The present invention relates to a plasma display device known as a display device.
Recently, expectations have run high for large-screen, wall-hung televisions as interactive information terminals. There are many display devices for those terminals, including a liquid crystal display panel, a field emission display and an electroluminescent display, and some of these devices are commercially available, while the others are under development. Of these display devices, a plasma display panel (hereinafter referred to as xe2x80x9cPDPxe2x80x9d) is a self-emissive type and capable of beautiful image display. Because the PDP can easily have, for example, a large screen, the display using the PDP has received attention as a thin display device affording excellent visibility and has increasingly high definition and an increasingly large screen.
The PDP is broadly classified as an AC or DC type according to its driving method and classified as a surface discharge type or an opposing discharge type according to its discharge form. In terms of high definition, large screen size and facilitation of production, the surface discharge AC type PDP has become mainstream under present conditions.
FIG. 5 illustrates an example of the structure of a conventional PDP. As shown in FIG. 5, the PDP is constructed of front panel 1 and back panel 2. Front panel 1 is constructed by forming a plurality of stripe-shaped display electrodes 6 each formed of a pair of scan electrode 4 and sustain electrode 5 on transparent front substrate 3 such as a glass substrate, covering display electrodes 6 with dielectric layer 7, and forming protective film 8 made of MgO over dielectric layer 7. Scan electrode 4 and sustain electrode 5 are formed of respective transparent electrodes 4a, 5a and respective bus electrodes 4b, 5b, formed of Crxe2x80x94Cuxe2x80x94Cr, Ag or the like, and which are electrically connected to respective transparent electrodes 4a, 5a. A plurality of black stripes or light-shielding films (not shown) is each formed between display electrodes 6 and is parallel to these electrodes 6.
Back panel 2 is constructed by forming address electrodes 10 in a direction orthogonal to display electrodes 6 on back substrate 9, which is disposed to face front substrate 3, covering address electrodes 10 with dielectric layer 11, forming a plurality of stripe-shaped barrier ribs 12 parallel to address electrodes 10 on dielectric layer 11 with each barrier rib 12 located between address electrodes 10, and forming phosphor layer 13 between barrier ribs 12 so that this layer 13 covers a side of each barrier rib 12 and dielectric layer 11. Typically, red, green and blue phosphor layers 13 are successively deposited for display in color.
Substrates 3, 9 of front and back panels 1, 2 are opposed to each other across a minute discharge space with display electrodes 6 orthogonal to address electrodes 10, and their periphery is sealed with a sealing member. The discharge space is filled with discharge gas, which is made by mixing for example, neon and xenon, at a pressure of about 66,500 Pa (500 Torr). In this way, the PDP is formed. The discharge space of the PDP is partitioned into a plurality of sections by barrier ribs 12, and display electrodes 6 are provided to define a plurality of discharge cells or light-emitting pixel regions between barrier ribs 12. Display electrodes 6 are disposed orthogonal to address electrodes 10.
FIG. 6 is a plan view detailing the structure of the discharge cell formed by display electrode 6 and barrier ribs 12. As shown in FIG. 6, display electrode 6 is formed by disposing scan electrode 4 and sustain electrode 5 with discharging gap 14 between electrodes 4, 5. Light-emitting pixel region 15 is a region surrounded by this display electrode 6 and barrier ribs 12, and non-light-emitting region 16 is present between adjacent display electrodes 6 of the discharge cells. With this PDP, discharge is caused by periodic application of voltage to address electrode 10 and display electrode 6, and ultraviolet rays generated by this discharge are applied to phosphor layer 13, thereby being converted into visible light. In this way, an image is displayed.
Higher luminance, higher efficiency, lower power consumption and lower cost are demanded of the plasma display device. To achieve higher efficiency, discharge in the part shielded from the light needs to be minimized by controlling the discharge. For example, Japanese Patent Unexamined Publication No. H8-250029 discloses a method for improving the efficiency. According to this known method, light emission in a part masked by a metal row electrode not transmitting the light is suppressed by increasing the thickness of a dielectric above this metal row electrode.
In the above-described conventional structure, to suppress the light emission in the part where the dielectric has the increased thickness, the dielectric needs to be increased to such a thickness as to allow enough suppression of the discharge. However, this increases the distance between the display electrode and the address electrode of the back substrate, whereby the voltage may rise in addressing.
There is a method of increasing numerical aperture for increasing another efficiency, that is, efficiency of extraction of the light from the phosphor. Since the bus electrode is made of metal, which does not transmit the light, for the purpose of reducing resistance of the electrode of the front substrate, the numerical aperture decreases. Increasing the extraction efficiency for this reason requires increasing the distance between the bus electrode and the light-emitting region as much as possible. However, this reduces the distance between the respective parallel electrodes of the adjacent cells, thus causing easy charge transfer between the adjacent cells. Accordingly, so-called crosstalk occurs, resulting in the cell undesirably emitting the light. Consequently, display quality reduces considerably.
Since the dielectric above the metal electrode needs to be increased to enough thickness for suppression of the discharge above this metal electrode, the voltage rises in addressing even in this case. If the dielectric does not have enough thickness, the crosstalk cannot be suppressed.
The present invention addresses such problems and aims to improve the efficiency and image quality.
To address the problems discussed above, a plasma display device of the present invention has the following structure. The plasma display device includes a pair of front and back substrates opposed to each other to form between the substrates a discharge space partitioned by a barrier rib, a plurality of display electrodes each disposed on the front substrate to form a discharge cell between the barrier ribs and a dielectric layer formed above the front substrate to cover the display electrodes, and a phosphor layer which emits light by discharge between the display electrodes. The dielectric layer is constructed of at least two layers of different dielectric constants and is formed with, at a surface thereof closer to the discharge space, a recessed part in each of the discharge cells.
According to the present invention, forming the recessed part in the dielectric layer increases capacitance in the recessed part, whereby charges concentrate on a bottom of the recessed part during their formation. Accordingly, a discharge region is limited, and consequently, highly efficient discharge can be realized. The structure having the two layers of different dielectric constants can suppress crosstalk even if this structure has reduced thickness.